This invention relates to a selective call signal detection circuit for selective high-speed detection of radio call signals at a multi-channel access radio receiving station, and more particularly to a selective call signal detection circuit for quickly discerning and detecting selective call signals at a mobile radio receiving station of the circulation non-stationary type, among other types of the multi-channel access systems.
Among mobile radio systems in recent use is one called a multi-channel access system. For the purpose of ensuring efficient use of radio channels, the individual receiving stations of this system share a plurality of channels from among which one not in use is automatically selected. The following methods have been so far devised for a given station in this system to put a call through to a desired receiving station.
(1) Call channel method--In this method a specific channel is assigned exclusively for calls so that mobile stations, while not engaging in communication, wait for calls through this specific channel.
(2) Circulation stationary method--In this method the base station keeps transmitting free line signals through a free channel and unengaged mobile stations wait for calls through this particular free communication channel.
(3) Circulation non-stationary method--In this method unengaged mobile stations sequentially scan all the channels and the transmitting station issues a selective call signal to a specific receiving station through any one of the unengaged channels. Each receiving station therefore continues to scan the channels until it identifies a channel which happens to be carrying a selective call signal addressed to themselves, whereafter it engages in communication through the channel.
This invention is directed to improvements in and concerning the third mentioned method.
Now, the method for selective call signal detection adopted conventionally in the receiving unit of this circulation non-stationary method (3) will be described. The receiving station continues sequentially scanning all the shared channels while discriminating between unengaged channels (channels in "free" state) and engaged channels (channels in "busy" state), and checking each of the channels in the "busy" state for a length of time proportional to the length of the selective call signal to determine whether or not the channel being checked is carrying its own selective call signal.
The most serious disadvantage of this conventional method of selective call signal discrimination resides in the fact that this method makes absolutely no discrimination between the "busy" channels which are already engaged in communication in the general sense of the word and "busy" channels through which selective call signals are currently in transmission. As regards the channels which are being used for general communication, this method wastes time checking these channels in spite of the fact that they never carry selective call signals.
This disadvantage will become more conspicuous when the description is given more specifically with numerical values.
Generally, a review of selective call signals in terms of composition or data format reveals that the call signals now in use consist of as many as 115 bits even in relatively short compositions. Specifically, the first 16 bits are bit synchronizing signals, the following 15 bits frame synchronizing signals, and the remaining 84 bits data proper to individual stations. The data transfer rate, even in a relatively rapid type, is 1200 bps. Under the conditions, the reading of one selective call signal requires at least 115/1200=95.8 ms of time. Moreover, in actuality, the receiving station does not always start receiving a given selective call signal from the very beginning of the bit synchronizing signal part thereof at the time that the receiving station switches itself to the particular channel carrying that selective call signal. Instead, there are times when the transmission of data has already been started at the moment that the receiving station is switched to that particular channel. The time required by the receiving station in completing full test of one channel, therefore, is twice the length of the aforementioned selective call signal, namely a time slightly over 190 ms.
The time for switching from one channel to another normally takes slightly over 8 ms. It is seen, therefore, that the time required for detailed checking of one channel is about 200 ms.
Let us assume a system wherein, out of a total of 40 channels, 20 channels are in the "busy" state and only 5 of these 20 busy channels are being used for transmitting selective call signals. This means that of the 20 "busy" channels, 15 are already being used for normal communication.
Under this condition, the conventional method as already described treats the channels already used for normal communication and the channels currently conveying selective call signals equally as channels in the "busy" state and merely discriminates these "busy" channels from the channels in the "free" state. If the time required for the discrimination between the free/busy states is assumed to be 5 ms, the time required for one complete cycle for sweeping all the channels will be found by the following calculation to be slightly over 4 seconds: EQU 200 (ms).times.20+[(8+5)] (ms).times.20=4260 (ms)
As is evident from the foregoing description, in the first term of the lefthand member of the equation, the time spent on the 15 channels, namely about 3 seconds' time, is clearly wasted. If the method were capable of discriminating between the group of channels which are already being used for normal communication and the group of channels currently conveying selective call signals, the foregoing equation would be altered to the following form: EQU 200 (ms).times.5+[(8+5)] (ms).times.35=1455 (ms)
In this case, therefore, the time required for one complete cycle for sweeping the channels will be found to be slightly below one and a half seconds.